I. Technical Field
The present invention pertains generally to telecommunications, and particularly to a High Speed Downlink Packet Access (HSDPA) system such as that operated (for example) in a Universal Mobile Telecommunications System (UMTS) terrestrial radio access network (UTRAN).
II. Related Art and Other Considerations
In a typical cellular radio system, mobile terminals (also known as mobile stations and mobile user equipment units (UEs)) communicate via a radio access network (RAN) to one or more core networks. The user equipment units (UEs) can be mobile stations such as mobile telephones (“cellular” telephones) and laptops with mobile termination, and thus can be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data with radio access network.
The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by a unique identity, which is broadcast in the cell. The base stations communicate over the air interface (e.g., radio frequencies) with the user equipment units (UE) within range of the base stations. In the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a radio network controller (RNC). The radio network controller, also sometimes termed a base station controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM), and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology.
As wireless Internet services have become popular, various services require higher data rates and higher capacity. Although UMTS has been designed to support multi-media wireless services, the maximum data rate is not enough to satisfy the required quality of services.
In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. One result of the forum's work is the High Speed Downlink Packet Access (HSPA). The High Speed Packet Access (HSPA) enhances the WCDMA specification with High Sped Downlink Packet Access (HSDPA) in the downlink and Enhanced Dedicated Channel (E-DCH) in the uplink. These new channels are designed to support IP based communication efficiently, providing enhanced end-user performance and increased system capacity. Though originally designed for interactive and background applications, they provide as good or even better performance for conversational services than the existing CS bearers.
Concerning High Speed Downlink Packet Access (HSDPA) generally, see, e.g., 3GPP TS 25.435 V6.2.0 (2005-06), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; UTRAN Iub Interface User Plane Protocols for Common Transport Channel Data Streams (Release 6), which discusses High Speed Downlink Packet Access (HSDPA) and which is incorporated herein by reference in its entirety. Also incorporated by reference herein as being produced by the forum and having some bearing on High Speed Downlink Packet Access (HSDPA) or concepts described herein include: 3GPP TS 25.425 V6.2.0 (2005-06), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; UTRAN Iur interface user plane protocols for Common Transport Channel data streams (Release 6); and 3GPP TS 25.433 V6.6.0 (2005-06), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; UTRAN Iub interface Node B Application Part (NBAP) signaling (Release 6).
High Speed Downlink Packet Access (HSDPA) is also discussed in one or more of the following (all of which are incorporated by reference herein in their entirety):
U.S. patent application Ser. No. 11/024,942, filed Dec. 30, 2004, entitled “FLOW CONTROL AT CELL CHANGE FOR HIGH-SPEED DOWNLINK PACKET ACCESS”;
U.S. patent application Ser. No. 10/371,199, filed Feb. 24, 2003, entitled “RADIO RESOURCE MANAGEMENT FOR A HIGH SPEED SHARED CHANNEL”;
U.S. patent application Ser. No. 11/292,304, filed Dec. 2, 2005, entitled “Flow Control For Low Bitrate Users On High-Speed Downlink”;
PCT Patent Application PCT/SE2005/001247, filed Aug. 26, 2005; and
PCT Patent Application PCT/SE2005/001248, filed Aug. 26, 2005.
HSDPA achieves higher data speeds by shifting some of the radio resource coordination and management responsibilities to the base station from the radio network controller. Those responsibilities include one or more of the following (each briefly described below): shared channel transmission, higher order modulation, link adaptation, radio channel dependent scheduling, and hybrid-ARQ with soft combining.
In shared channel transmission, radio resources, like spreading code space and transmission power in the case of CDMA-based transmission, are shared between users using time multiplexing. A high speed-downlink shared channel is one example of shared channel transmission. One significant benefit of shared channel transmission is more efficient utilization of available code resources as compared to dedicated channels. Higher data rates may also be attained using higher order modulation, which is more bandwidth efficient than lower order modulation, when channel conditions are favorable.
The radio base station monitors for the channel quality (CQI) of the high-speed downlink shared channel (HS-DSCH) and manages a priority queue maintained at the radio base station. The base station's priority queue (PQ) stores data which is to be sent on the high-speed downlink shared channel (HS-DSCH) over the air interface to the mobile terminal. In addition, knowing from the monitor the carrier quality of the HS-DSCH, the base station sends to the control node messages which authorize the control node to send more HS-DSCH data frames to the radio base station.
The mobile terminal reports a channel quality indicator (CQI) to the radio base station in charge of the cell. The CQI is a measure of the quality of the common pilot CPICH as reported by each mobile station (e.g., each user equipment unit (“UE”)). The channel quality indicator (CQI), together with an expression(s) of capabilities of the mobile terminal, is translated to a bitrate. The bitrate is then further reduced if needed by the radio base station, which results in generation of capacity allocation control frames which are sent to the control node regularly and/or per need bases, e.g. at urgent transitions. The authorizing messages include a “capacity allocation” which can be expressed in various ways, such as in terms of either bitrate or credits, for example. For example, capacity allocation expressed in credits may refer to a number of MAC-d PDUs that the radio network controller (RNC) is allowed to transmit for the MAC-d flow. In response to these authorizing messages, the control node sends further HS-DSCH frames to the radio base station.
The data in the priority queues is sent from a control node to a radio base station in protocol data units (PDUs). A number of PDUs may be included in each high-speed downlink shared channel (HS-DSCH) data frame.
Thus, HSDPA is a shared channel designed for efficient support of packet data applications. Enhancements over dedicated (and shared) channels include fast link adaptation; fast scheduling; Hybrid ARQ from Node B; and a short transmission time interval (TTI). In terms of fast link adaptation, the link adaptation is done by selecting the best modulation and coding scheme based on channel quality indicator from the UE. For fast scheduling, the selection of the user is done in the Node B, which has access to the link quality information, and thus can select the optimal user. Hybrid ARQ from Node B involves having a retransmission mechanism in the base station which allows fast retransmissions and quick recovery of erroneous link adaptation decisions. As a short TTI, a two millisecond (ms) TTI is used for all transmissions.
E-DCH is dedicated channel that has been enhanced for IP transmission. Enhancements include the possibility of using use a shorter TTI; fast hybrid ARQ (HARQ) between mobile terminal and the base station; scheduling of the transmission rates of mobile terminals from the base station; and the fact that E-DCH retains majority of the features characteristic for dedicated channels in the uplink. In terms of fast hybrid ARQ (HARQ) between mobile terminal and the base station, the HARQ mechanism is semi-persistent, as it will abandon a transmission after a fixed number of transmission attempts. The number of transmission attempts is signaled from the RNC to the UE.
Since the uplink transmissions are not orthogonal, E-DCH is power controlled in order to avoid creating excessive interference that might make it impossible to detect other users' signals. The power control comprises two different mechanisms. The first is a inner loop power control which is located in the base station node and which is performed for each ⅔ ms slot. In the inner loop power control, the transmitted power is adjusted so that the measured received signal strength of the Dedicated Physical Control Channel (DPCCH) reaches a predefined signal-to-interference ratio (SIR) target. This target is determined by a second mechanism, e.g., an outer loop power control, which tries to maintain a consistent block error rate for selected transmission attempt. The outer loop power control is located at a radio network control (RNC) node.
For delay-critical services such as VoIP, one general system goal is to keep delay within preconfigured boundaries. Rather than to increase transfer delay, the system therefore uses different mechanisms to drop packets when link problems occur. In this regard, both HSDPA and E-DCH can drop PDUs after selected number of retransmissions.
For HSDPA there are several mechanisms that can result in a dropped packet. These mechanisms include one or more of reordering timer(s); delay schedulers; and limited number of retransmissions.
As one mechanism that can result in a dropped packet, a reordering timer (T1 timer) may be utilized to provide or ensure in-order delivery of packets. The UE abandons a PDU if it is not received when the T1 timer expires. Similarly, the Node B will stop transmitting a packet if it has not been received before T1 timer expires.
For voice traffic, it is expected that a second mechanism—a specific scheduler (“delay scheduler”)—is used. This scheduler has a settable limit on the maximum queuing delay, after which the packets are dropped in the Node B. As a third and likely future mechanism, there may be a limit on the maximum number of retransmissions.
As with HSDPA, packets can also be dropped for the E-DCH. In particular, a unit in the medium access control (MAC) layer which is responsible for the E-DCH (e.g., a MAC-e entity) in the UE will drop packets after a preconfigured number of transmission attempts. A preconfiguration including the preconfigured number is signaled to the User Equipment (UE) from the network using Radio Resource Control (RRC) protocol, and all standard conforming UEs will need to implement this limitation. While there may be other reasons (such as misinterpretation of the protocol feedback) for packet loss, it is expected that operation which exceeds the number of transmission attempts will be the dominating reason.
For conversational services, it is expected that MAC-e and MAC-hs are the only protocols responsible for performing retransmissions, and so all packet losses on MAC-e and MAC-hs layers will result in application layer packet loss.
For various applications, consecutive packet losses are more harmful than isolated packet losses. Examples of such applications are voice (and other real time) applications and applications based on Transmission Control Protocol (TCP). For voice applications, error concealment can often hide individual packet losses so that the user does not even notice it. However, (sufficiently many) consecutive packet losses cannot be repaired and can lead to noticeable impairments in speech. For these applications the link layer should actually try to minimize the consecutive packet losses. Most TCP based applications can also recover from a single isolated packet loss, but many will create a time-out after just two consecutively lost packets.
Thus, the existing MAC-e and MAC-hs implementations may encounter situations, in which they will either occasionally or typically drop packets, resulting in degraded application performance. Some typical reasons and/or scenarios for packet dropping are discussed below:
In the downlink, the UE may report incorrect channel quality to the base station. This can be expected to happen at the beginning of the transmission, when the UE has not been able to measure the own-signal interference. If this happens, Node B will incorrectly use too low a power, resulting in more than expected transmissions being needed. These extra retransmissions may need more retransmissions than the default T1 timer setting allows. The Node-B has access to the retransmission sequence number, which indicates how many retransmissions have been made for a particular PDU. The signaling is not absolutely necessary, but may help (to avoid unnecessary packet discarding in the reordering functionality. Generally increasing T1 timer is not an option, as especially for conversational applications it is important to keep the value of the T1 timer low in order to allow quick delivery of subsequent packets after a packet loss not caused by poor radio environment. Similar arguments apply also for delay threshold in the scheduler. Delay and T1 settings are based on the delay attribute negotiated for the radio access bearer in question.
In the downlink, the UE may be located in a difficult radio environment, and more than a normal number of retransmissions may be needed, resulting again in T1 timer or delay threshold expiring.
In the uplink, the targeted transmission power is determined by the outer loop power control mechanism. Typically the outer loop power control reduces the transmission power until a block error is observed, and then increases the power again. Due to delay in the control loop, more than one packet is often lost before the target power is raised again.
In the uplink, the UE may not have sufficient transmission power to maintain the current data rate. In this case, the UE may either (1) reduce data rate by transmitting fewer bits per transmission time interval (TTI), or transmit with reduced (insufficient) power (if reducing the number of bits is not possible (e.g. at the cell border, or due to chosen configuration)). Reducing data rate by transmitting fewer bits per transmission time interval (TTI) result in packets being queued in the RLC buffers. Once the buffer is full, a typical implementation will simply drop all incoming packets. When transmitting with reduced (insufficient) power, more retransmissions are needed to transmit the packet. It is possible that the number of retransmissions needed is greater than the configured maximum number of transmissions.
In cases wherein a number of consecutive packets are dropped, such consecutive packet dropping is likely to have a large effect on the application performance.
What is needed therefore, and an object of the present invention, are apparatus, methods, and techniques for better handling packet loss/delay conditions in a telecommunications system.